In recent display devices represented by liquid crystal monitors, transparent conductive films are largely used for electrodes. These films need to have a low electrical resistivity and a high transmittance of light in a visible region of wavelengths. As substances satisfying such characteristics in good balance, metal oxide thin films are chiefly used at present, and tin oxide (SnO2) base (which is mainly utilized by doping F or antimony (Sb)) having high chemical stability, indium oxide (In2O3), and tin-doped indium oxide (In2O3—SnO2, which is hereinafter referred to as ITO) having excellent electrical and optical properties are known.
However, new display devices, such as organic or inorganic EL elements and electronic papers, have recently been developed and requirements for transparent conductive films have been diversified so that common ITO crystal films can no longer accommodate the requirements.
For example, there is a problem that when the ITO crystal film is used for the organic EL element, the local concentration of electric current is produced because a projection structure due to crystal growth is present, and uniform display becomes difficult. In the visible region, the transmittance of light in a short-wavelength region (the visible region of short wavelengths) of about 380-400 nm is particularly low, and thus there is another problem that the efficiency for taking out the light of a particular wavelength from a light-emitting layer is impaired. From these problems, it is demanded that an amorphous transparent conductive film whose surface is extremely flat or a transparent conductive film having a high transmittance of light in the visible region, notably in the visible region of short wavelengths, should be used for the organic EL element.
As another example, a transparent conductive film that is hard to break with respect to bending is essential for the electronic paper characterized by flexibility. In general, an oxide crystal film has a grain boundary that is slight in structure and is liable to break. Thus, it is known that an amorphous film which is free of the grain boundary is hard to break with respect to bending. From this, it is proposed to use the amorphous transparent conductive film as the transparent conductive film that is resistant to bending. It is needless to say that the high transmittance in the visible region of short wavelengths is important for the amorphous transparent conductive film, like the organic EL element, in order to improve the taking-out efficiency of light.
Patent Reference 1 described below proposes the organic electroluminescence element comprising an organic layer that contains an organic light-emitting layer, sandwiched between an anode and a cathode so that the cathode includes, in order from a side coming in contact with the organic layer, an electron injection electrode layer, a transparent conductive film, and a metallic thin film with a resistivity of 1×10−5Ω·cm or less, laminated one over another and a transparent thin film layer is formed outside the cathode. In this case, an amorphous transparent conductive film using an oxide composed of indium (In), zinc (Zn), and oxygen (O) is applied.
Patent Reference 2 described below sets forth a transparent conductive film in which a compound metallic oxide film containing In, Sn, and Zn, as the transparent conductive film having the properties of the high transmittance of visible light and low resistance, forms at least one kind of In4Sn3O12 crystal, or microcrystals or amorphism composed of In, Sn, and Zn, and as the composition of metal contained therein, an Sn content indicated by Sn×100/(In═Sn) is 40-60 at. % and a Zn content indicated by Zn×100/(In═Zn) is 10-90 at. %.
Patent Reference 3 described below proposes a transparent conductive film in which, in a quasi-two-dimensional system indicated by an oxide containing magnesium (Mg) and indium (In), MgO—In2O3, as the transparent conductive film having a band gap of 3.4 eV and a refractive index of light of 2.0 that are almost the same as in a conventional transparent conductive film and possessing much higher conductivity than MgIn2O4 and In2O3, namely lower resistivity and excellent optical properties, an In content indicated by In/(Mg═In) is 70-95 at. %.
Patent Reference 4 described below proposes a transparent conductive film in which, in a quasi-two-dimensional system indicated by Ga2O3—In2O3 as the transparent conductive film having a composition range considerably different from GaInO3 which has been known in the past and possessing much higher conductivity than GaInO3 and In2O3, namely lower resistivity and excellent optical properties, a Ga content indicated by Ga/(Ga═In) is 15-49 at. %.    Patent Reference 1: Japanese Patent Kokai No. Hei 10-294182    Patent Reference 2: Japanese Patent Kokai No. Hei 10-83719    Patent Reference 3: Japanese Patent Kokai No. Hei 8-264023    Patent Reference 4: Japanese Patent Kokai No. Hei 9-259640    Patent Reference 5: Japanese Patent Kokai No. Hei 7-182924
However, in each of the above conventional transparent conductive films, the problems still remain that the transmittance of light in the visible region of short wavelengths is low and the taking-out efficiency of light mentioned above is impaired.
Patent Reference 5 states that, in a gallium-indium oxide (GaInO3) film in which a heterovalent dopant like a quadrivalent atom is doped by a small amount, transparency is promoted, index matching is improved, and the electrical conductivity of nearly the same level as in a wide-band-gap semiconductor used at present can be attained. This film is capable of bringing about the high transmittance in the visible region of short wavelengths, but has still the drawbacks that, as in the crystal film mentioned above, the projection structure is produced on the surface of the film and the film is liable to break with respect to bending. Moreover, a substrate temperature as high as about 350° C. is required and thus is disadvantageous to a fabrication process.
Therefore, there is nothing to sufficiently accommodate, in good balance, the demand for the amorphous transparent conductive film whose surface is extremely flat in the organic EL element, the demand for the high transmittance in the visible region, notably in the short-wavelength region (in the visible region of short wavelengths) of about 380-400 nm, and the need for the transparent conductive film that is hard to break with respect to bending in the electronic paper characterized by flexibility. The development of the transparent conductive film combining these properties has been strongly desired.